[Solved]-using LINQ to process a text file-LINQ,C#

Linq probably isn't your best bet here. Just try doing

var lines = File.ReadAllLines(filename);
List<string> linesICareABout = new List<string>();
for(int i = 0; !linesICareAbout[i].Contains("part2=002"); ++i)
{
 linesICareABout.Add(lines[i]);
}

Then do whatever you want with the lines you read in.

However, if you're really dedicated to using Linq, try TakeWhile

http://msdn.microsoft.com/en-us/library/bb534804.aspx

The simplest and streghtforward solution comes to me is something like this:

var lines = File.ReadAllLines(@"C:\Sample.txt").
             SkipWhile(line=>!line.Contains("part1")).
                   Skip(1).TakeWhile(line=>!line.Contains("part2"));

It returns result you want actually. The logic is simple:

• SkipWhile lines till meeting a line that contains "part1"
• after Skip(1) (as it will be actually that one that contains "part1" string)
• finally Take those ones till getting to the line containing "part2".

public static IEnumerable<string> GetLinesBetween(
    string path,
    string fromInclusive,
    string toExclusive)
{
    return File.ReadLines(path)
        .SkipWhile(line => line != fromInclusive)
        .TakeWhile(line => line != toExclusive);
}

var path = Path.Combine(sExecPath, sFileName); // don't combine paths like that
var result = GetLinesBetween(path, "part1=001", "part2=002").ToList();

I think you are looking for TakeWhile:

var linesInPartOne = File
       .ReadAllLines(sExecPath + @"\" + sFileName)
       .SkipWhile(line => !line.StartsWith("**part1="))
       // To skip to part 1 header line, uncomment the line below:
       // Skip(1)
       .TakeWhile(line => !line.StartsWith("**part2="));

To generalize this to retrieve any given numbered part, something like this would do:

public static IEnumerable<String> ReadHeaderPart(String filePath, int part) {
    return File
        .ReadAllLines(filePath)
        .SkipWhile(line => !line.StartsWith("**part" + part + "="))
        // To skip to part 1 header line, uncomment the line below:
        // Skip(1)
       .TakeWhile(line => 
            !line.StartsWith("**part" + (part + 1) + "=" 
            && 
            !line.StartsWith("end_header")))
       .ToList();
 }

EDIT: I had a Skip(1) in there to skip the part 1 header. Removed it since you seem to want to keep that line.